;递归法求加法
(define (c a b)
(if (= a 0)
b
(1+ (c (- a 1 ) b))))
;( c 3  4)
;( +1 ( c 2 4 ))
;(+1 (+1 (c 1 4 )))
;(+1 (+1 (+1 (c 0 4))))
;(+1 (+1 (+ 1 4)))
;(+1 (+1 5))
;(+1 6)
;7